System, method, and apparatus for non-intrusively determining concentration of a solute in a solution

ABSTRACT

Concentration of a solute in a solution is determined. Light from a light source is received at a chamber containing the solute and the solution. The light is transmitted along an optical path length of the chamber, through the solute and the solution, and output from the chamber. The light output from the chamber is detected by a detector. The optical path length of the chamber is selected to optimize sensitivity of the detector. The concentration of the solute in the solution is determined based on the light received by the detector.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/293,976 filed May 30, 2001.

Portions of this invention were made with Government support (ContractNos. 0CAST 5662 and 5833). The Government may have certain rights inthis invention.

BACKGROUND

The present invention is directed to a system, method, and apparatus fordetermining concentration of a solute in a solution. In particular, thepresent invention is directed to a system, method, and apparatus fordetermining concentration of a solute in a solution in a non-intrusivemanner.

It is often desirable to be able to determine the concentration of asolute in a solution. For example, it is desirable to be able todetermine the concentration of chlorine in treated water.

Active chlorine content of treated water is typically 1-10 ppm, andnominally levels are less than 5 ppm. However, those levels may rise ashigh as 10 ppm during shock treatments.

Conventional commercial chlorine detectors have many drawbacks. Forexample, they do not produce results in real time and often requirehuman intervention to make interpretations based on color matching.Also, they typically employ a reagent that contaminates water supplies.Furthermore, they lack support electronics to control chemical feedand/or water quality control.

Most substances absorb radiation in the UV/VIS/NIR (UltraViolet/Visible/Near Infrared) regions of the electromagnetic spectrum.Each chemical species allows a specific amount of light of a givenwavelength to be transmitted, thus creating a “signature” for thatspecies due to the wavelength-dependent index of refraction. Bystrategically picking off peaks in absorption (attenuation of aparticular wavelength band), it is possible to classify and evenquantify various chemical species.

Theories have been posited for utilizing absorption spectra in the UVregion to detect concentration of a solvent in a solution. For example,“Water-core waveguide for pollution measurement in the deepultraviolet”, by Peter Dress et al. describes evaluation of theperformance of UV fibers and their degradation over time due toexcessive exposure. There is a significant absorption peak for activechlorine centered at 290 nm for a pH of 10.2, as observed in Dress etal. This peak can shift in wavelength depending on pH of the watersample. With decreasing pH, the absorption peak shifts to lowerwavelengths or higher energies, while the opposite effect is observedwith increasing pH. While Dress et al. suggests use of absorptionspectra to detect chlorine concentration, this paper presents datashowing poor results below 10 ppm.

In “Flow-injection chemiluminescence sensor for the determination offree chlorine in tap water”, by Wei Qin et al., chemiluminescence isexplained. One of the drawbacks of the method described in Qin et al. isthat it requires an injection of the reagent luminol. Also, Qin et al.reports a lack of sensitivity required for low concentrationmeasurement.

There is thus a need for a non-invasive and non-destructive techniquefor determining low level concentrations of a solute in a solution.

SUMMARY

It is therefore an object of the present invention to provide a system,method, and apparatus for detecting concentration of a solute in asolution in a non-invasive, non-destructive manner.

According to exemplary embodiment, this and other objects are met by asystem, method, and apparatus for determining concentration of a solutein a solution. Light from a light source is received at a chambercontaining the solute and the solution. The light is transmitted alongan optical path length of the chamber, through the solute and thesolution, and output from the chamber. The light output from the chamberis detected by a detector. The optical path length of the chamber isselected to optimize sensitivity of the detector. The concentration ofthe solute in the solution is determined based on the light received bythe detector.

The objects, advantages and features of the present invention willbecome more apparent when reference is made to the following descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates an exemplary system for detecting the concentrationof a solute in a solution;

FIG. 1B illustrates an exemplary implementation of elements of thesystem shown in FIG. 1A;

FIG. 2 illustrates an exemplary spectral distribution of a light source;

FIG. 3 illustrates an exemplary method for detecting soluteconcentration according to an exemplary embodiment; and

FIGS. 4A and 4B illustrate exemplary paths taken by incident lightthrough a chamber containing a solute in a solution.

DETAILED DESCRIPTION

According to an exemplary embodiment, a non-invasive, non-destructiveapproach has been developed for detecting low level concentration of asolute in a solution. An exemplary system for concentration detection isshown in FIG. 1A.

The system includes a sample chamber 120 that contains the solute, suchas chlorine, mixed with a solvent, such as water, in a solution. Thechamber 120 may be a pipe or tube that may be adjusted in length toadjust the optical path length. The pipe or tube may also be adjusted indiameter to allow more water to flow into the pipe as well as to providea larger cross sectional area.

Light from a light source is delivered to the sample chamber 120 viadelivery optics 110. The light is transmitted through the solution inthe chamber and delivered via exit optics 130 to a detector. Thedetector includes wavelength selective optics 140 and photosensitiveelements 150. The wavelength selective optics 140 filter out wavelengthsof light, including the peak absorption wavelengths for the solute, andthe photosensitive elements detect the intensity of detected light atthose wavelengths. The wavelength selective optics may include, e.g., adiffraction grating or filters, and the photosensitive elements mayinclude, e.g., photosensitive diodes. The detected intensity is fed toan analyzer that includes analog signal processing 160 and digitalsignal processing 170.

FIG. 1B illustrates in detail an example of implementation of elementsof the detection system shown in FIG. 1A. According to an exemplaryembodiment, the input light from the light source is broadband. Thewavelength band may change, depending on the absorption characteristicsof the solute. For example, for chlorine concentration detection, aflashed xenon arc lamp 105 may be used as the light source to generateradiation in the UV spectral region of interest. The xenon sourcecontains some intensity of all wavelengths over the range from 190 nm to3000 nm. Its spectral distribution is displayed in FIG. 2. The abilityto selectively pulse the lamp allows for greater flexibility whenselecting integration times for the detector.

The sample chamber 125 may be cylindrical and may be constructed ofpolyvinyl chloride (PVC) tubing with fused-silica windows. Water may bepassed through tubes attached to either end of the cylindrical chamber.The diameter of the tube may be chosen to match the size of the opticused to focus the output light onto the input slit of the detector,e.g., spectrograph.

According to one embodiment, the detector 180 includes a 256-elementdetector array 155 and a diffraction grating 145. The detector may becoupled to a parallel spectrograph that allows for simultaneousacquisition of reference and source signals. Light may be coupled intoan UV-enhanced fused-silica fiber 190 directly from the light source andfed into the spectrograph for use as a reference. As depicted in FIG.1B, half of the detector elements are dedicated to the reference signaland half to the source signal. The simultaneous sampling of source andreference signals make possible the elimination of noise contributed byfluctuations in the light source.

Although not shown in FIG. 1B, a driver/amplifier may be used as theanalyzer. The driver/amplifier may be custom built or an off-the-shelfC4070 driver/amplifier. The driver/amplifier may be a combination videosignal processor and control signal generator for the detector array.The reading of the detector may be similar to the method used with mostPhotodiode Arrays (PDA's) or Charge Coupled Devices (CCD's).

The C4070 performs a current integration on the video output of thedetector and generates a digital signal that is used to trigger dataacquisition. An analog-to-digital card, e.g., either a custom built cardor an off-the-shelf PC-based data acquisition card, receives the triggersignal and buffers a series of data points. PC software may be used toplot the data on a computer screen for evaluation and save it to anASCII text file for later processing. A DSP-based micro controller maybe used to regulate the integration time (with respect to the detectorintegration time referenced to the length of exposure) and control theflashing of the lamp.

The delivery optics and exit optics are not shown in FIG. 1B, in theinterest of simplifying the illustration. It will be appreciated thatthese optics may be implemented in any conventional manner.

FIG. 3 illustrates an exemplary method for determining concentration ofsolute in a solution. The method begins at step 300 at which lightsupplied by a light source is received at the chamber containing thesolute and the solution. At step 310, the received light is transmittedalong the optical path length of the chamber, through the solute and thesolution. At step 320, the transmitted light is output from the chamber.At step 330, the light output from the chamber is detected by thedetector at a sensitive determined by the optical path length. At step340, the concentration of the solute is determined based on the detectedlight.

Referring again to FIGS. 1A and 1B, the optical path length of thechamber 120 affects the sensitivity of the detector 180. For a givenintensity and molar absortivity of solute, e.g., chlorine, the opticalpath length determines the measurement range for the detector. As theoptical path length of the chamber gets longer, the measurement rangeshrinks, and the resolution of the detector goes up. If the optical pathlength gets shorter, then the measurement range increases, and theresolution becomes more course.

The amount of chlorine in water or the molar concentration may vary fromwater supply to water supply. Drinking water may have concentrations onthe order of parts per million (ppm). Therefore, detection of a fewparts per million is the necessary low end sensitivity level for thedetector for detection of chlorine concentration.

The relationship between the optical path length of the chamber and thedetector sensitivity may be represented by a generalized set ofequations. These equations may be derived by relating the signal tonoise ratio (SNR) of the detector type used to losses in the measurementmedia.

For two measurement wavelengths λ_(ref) and λ_(sig), where λ_(ref) is awavelength unaffected by the solute, and λ_(sig) is the peak absorptionwavelength of the solute, then a set of equations may be developed thatdepend only on optical path length and solute concentration for a givenspecies. Also, given absorption coefficients α for any solvent andsolute, it is possible to develop a set of generalized equations withthe same dependencies.

To simplify the derivation, the following assumptions are made:

1) The SNR is 10000:1 for photodiodes.

2) The light source has the same intensity at both measurementwavelengths

3) Light only passes through air and water. No optics are consideredhere, since the optics only add an integer loss.

4) The incident light intensity is high enough that with loss to waterthe reference intensity is large enough to saturate the detector.

To help understand the relationship between loss and sensitivity, FIGS.4A and 4B illustrate the paths taken by incident light through a solventin a chamber at the reference and signal wavelengths, respectively.

From FIG. 4A, it can be seen that the intensity of light leaving thesolvent at a wavelength of λ_(ref) may be given as:

 I _(ref) =I ₀ −I _(Wref)

where I₀ is the broadband incident light that is the same intensity forboth wavelengths, and I_(Wref) is the loss to water at λ_(ref). FromFIG. 4B, it can be seen that the intensity of light leaving the solventat a wavelength of λ_(sig) may be given as:

I _(sig) =I ₀ −I _(Wsig) −I _(Clsig)

where I_(Wsig) is the loss to water at λ_(sig), I_(Clsig) is the loss tochlorine at λ_(sig).

The Beer-Lambert Law states:

log₁₀(I ₀ /I)=ε*c _(m) *L

where ε is the molar absortivity, c_(m) is the molar concentration ofsolute, and L is the path length. Solving for I₀/I in terms of thenatural log (ln):

log₁₀(e ¹)*log_(e)(I ₀ /I)=ε*c _(m) *L

log₁₀(e ¹)=log₁₀(2.718)=0.4343

ln(I ₀ /I)=(ε*c _(m) *L)/0.4343

I ₀ /I=e ^((ε*cm*L)/0.4343)

The absorption coefficient α for a given solute may be given as:

α=(ε*c _(m))/0.4343

For chlorine,${{ɛ \approx {8.374\quad \frac{liters}{{Moles}*{centimeters}}\quad {and}\quad c_{m}}} = {c*\quad \frac{Moles}{17000\quad {mg}}}},$

where c is the concentration in parts per million (ppm) or$\frac{mg}{liter}.$

So for chlorine, the absorption coefficient α_(Cl) may be given as:$\alpha_{Cl} = {{\frac{8.374}{17000*0.4343}*c} = {0.001134*c}}$

From the Beer-Lambert Law, intensity losses can be written as:

I _(loss) =I ₀ *e ^((α*L)) −I ₀

Calculating the losses in FIG. 4A, then:

 I _(ref) =I ₀(2−e ^((α(w1)*L)))

where the α(w1) is the absorption coefficient for water at λ_(ref).

For the solute, using FIG. 4A:

I _(sig) =I ₀(3−e ^((α(Cl)*L)) −e ^((α(w2)*L)))

where α(w2) is the absorption coefficient for water at λ_(sig) and α(Cl)is the absorption coefficient for chlorine at λ_(sig).

Taking the SNR as 10000:1 for the photodiodes, if 10000 is defined to bethe maximum possible signal readable by a detector element (thesaturation charge), and I_(ref)=10000, the measurement range may berepresented as follows:

Thus, the measurement range is defined by the maximum signal intensityand the minimum signal intensity resolvable for a particular detectortype. Taking I_(sig)=1 for a given path length L, then:

1=10000(3−e ^((α(Cl)*L)) −e ^((α(w2)*L)))

0.0001=3−e ^((α(Cl)*L)) −e ^((α(w2)*L))

2.9999=e ^((α(Cl)*L)) +e ^((α(w2)*L))

 ln(2.9999)=α(Cl)*Lα(w 2)*L

1.09858/L−α(w 2)=α(Cl)

Inserting the equation for α(Cl), then:

1.09858/L−α(w 2)=0.001134*c _(max)

Solving for c:

c _(max)=1.09858/(L*0.001134)−α(w 2)/0.001134

c _(max)=968.8/L−α(w 2)*881.8

Thus, given a path length and absorption coefficient for the solvent atλ_(sig), the max measurable concentration c_(max) may be defined.

The smallest resolvable signal can be found by the following equation:

min signal=c _(max)/10000(2−e ^((α(w2)*L)))

Applying these equations to the detection of chlorine:

L=20.32 cm

α(w 2)=0.0036 cm⁻¹

c _(max)=968.8/L−α(w 2)*881.8=50.9 ppm

min signal=50.9/10000(2−e ^((α(w2)*L)))=0.0055 ppm

Thus, the range would be 0 to 50.9 ppm with increments of 0.0055 ppm.

According to exemplary embodiments, a non-intrusive solute concentrationdetection system has been developed that solves contamination problemsand improves the accuracy of current sensors. The system may be used fordirect insitu measurements of chlorine in water, reducing chemical useand lowering the overall cost of water treatment. The algorithmimplemented on the system utilizes multi-point data. This eliminateserror due to small fluctuations as seen with single point methods.

The system discussed herein is designed to further demonstrate that thephotometric process used can overcome these difficulties and that it canaccurately quantify active-chlorine concentrations in real time. Byappropriately setting the optical path length of the chamber, it ispossible to detect chlorine in the amounts of a few parts per million.

It should be understood that the foregoing description and accompanyingdrawings are by example only. A variety of modifications are envisionedthat do not depart from the scope and spirit of the invention.

For example, although detection of chlorine in water has been describedabove, the system may be used to detect the concentration of any solutein a solution that has significant absorption in the UV, visable or nearIR region.

The above description is intended by way of example only and is notintended to limit the present invention in any way.

What is claimed is:
 1. A system for determining concentration of asolute in a solution, the system comprising: an optical source forproviding light in a predetermined wavelength range; a chamber forcontaining the solute in the solution, the chamber having an input forreceiving the light, an optical path length along which the light istransmitted through the solute and the solution in the chamber, and anoutput for outputting the light transmitted through the chamber; adetector for receiving light transmitted along the optical path lengthof the chamber to the output of the chamber, wherein the optical pathlength of the chamber is selected to optimize sensitivity of thedetector; and an analyzer for determining the concentration of thesolute in the solution based on the light received by the detector,wherein the optical path length of the chamber is related to thesensitivity of the detector by the equations: min signal=c_(max)/10000(2−e ^((α(w2)*L))) c _(max)=968.8/L−α(w 2)*881.8 where minsignal represents the minimum resolvable signal, c_(max) represents themaximum detectable concentration, and L represents the optical pathlength of the chamber.
 2. The system of claim 1, wherein the opticalpath length of the chamber is selected to provide an adequate intensityof light to the detector.
 3. The system of claim 1, wherein the opticalpath length of the chamber is selected to provide an adequate resolutionfor the detector.
 4. The system of claim 1, wherein the analyzeranalyzes the spectral distribution of the received light and determinesthe concentration of the solute within the solution based on thespectral distribution.
 5. The system of claim 1, wherein the detectorincludes an array of detectors.
 6. The system of claim 1, wherein thedetector determines based on a signature of the received light theconcentration of the solute.
 7. The system of claim 6, wherein thesignature includes at least one peak absorption wavelength, and thedetector includes a detector for each peak absorption wavelength.
 8. Thesystem of claim 1, wherein the detector includes a wavelength selectiveoptics and photosensitive elements, and the analyzer analyzes the lightdetected by the photosensitive elements to determine the concentrationof the solute in the solution.
 9. The system of claim 8, wherein thewavelength selective optics include a diffraction grating.
 10. Thesystem of claim 8, wherein the wavelength selective optics includefilters.
 11. The system of claim 8, wherein the photosensitive elementsinclude photosensitive diodes.
 12. The system of claim 1, wherein thesolute is chlorine, and the solution is chlorine in water.
 13. Thesystem of claim 1, wherein the chamber includes an input for thesolution filling the chamber and output for outputting the solution. 14.The system of claim 13, wherein the flow of solution in and out ofvolume is controlled.
 15. The system of claim 1, further comprising ananalog and digital signal processing for comparing the received light ofthe detector with a reference signal.
 16. The system of claim 15,wherein the reference signal is fed directly from the optical source tothe detector.
 17. The system of claim 15, wherein the analog and digitalsignal processing subtracts the reference signal from the detectedsignal to eliminate noise.
 18. The system of claim 1, wherein theconcentration of solute in the solution is determined in real time. 19.The system of claim 1, wherein the optical light source includes acontrolled flashing light source.
 20. The system of claim 1, wherein thedetector sensitivity is in the parts per million (ppm) range.
 21. Amethod for determining concentration of a solute in a solution, themethod system comprising: receiving light in a predetermined wavelengthrange from a light source at a chamber including the solute in thesolution; transmitting the received light along an optical path lengthof the chamber, through the solute in the solution in the chamber;outputting the transmitted light from the chamber; detecting the outputlight, wherein the optical path length of the chamber is selected tooptimize sensitivity of the detection; and determining the concentrationof the solute in the solution based on the light received by thedetector, wherein the step of detecting is performed by wavelengthselective optics and photosensitive elements, wherein the step ofdetermining includes analyzing the light detected by the photosensitiveelements to determine the concentration of the solute in the solution,wherein the photosensitive elements include photosensitive diodes, andwherein the optical path length of the chamber is related to thesensitivity of the detector by the equations: min signal=c_(max)/10000(2−e ^((α(w2)*L))) c _(max)=968.8/L−α(w 2)*881.8 where minsignal represents the minimum resolvable signal, c_(max) represents themaximum detectable concentration, and L represents the optical pathlength of the chamber.
 22. The method of claim 21, wherein the opticalpath length of the chamber is selected to provide an adequate intensityof light to the detector.
 23. The method of claim 21, wherein theoptical path length of the chamber is selected to provide an adequateresolution for the detector.
 24. The method of claim 21, wherein thestep of determining includes analyzing the spectral distribution of thereceived light and determining the concentration of the solute withinthe solution based on the spectral distribution.
 25. The method of claim21, wherein the step of detecting is performed by an array of detectors.26. The method of claim 21, wherein the step of determining includeddetermines based on a signature of the received light the concentrationof the solute.
 27. The method of claim 26, wherein the signatureincludes at least one peak absorption wavelength, and the step ofdetecting includes detecting each peak absorption wavelength.
 28. Themethod of claim 21, wherein the wavelength selective optics include adiffraction grating.
 29. The method of claim 21, wherein the wavelengthselective optics include filters.
 30. The method of claim 21, whereinthe solute is chlorine, and the solution includes chlorine in water. 31.The method of claim 21, wherein the chamber includes an input for thesolution filling the chamber and output for outputting the solution. 32.The method of claim 31, wherein the flow of solution in and out ofvolume is controlled.
 33. The method of claim 21, further comprisingcomparing the received light with a reference signal.
 34. The method ofclaim 33, wherein the reference signal is fed directly from the opticalsource to the detector.
 35. The method of claim 33, wherein the step ofcomparing comprises subtracting the reference signal from the detectedsignal to eliminate noise.
 36. The method of claim 21, wherein the stepsare performed in real time.
 37. The method of claim 21, wherein theoptical light source includes a controlled flashing light source. 38.The method of claim 21, wherein the detector sensitivity is in the partsper million (ppm) range.
 39. An apparatus containing a solute in asolution, the apparatus comprising: a volume for containing the solutein the solution; an input for receiving light in a predeterminedwavelength range from a light source; an optical path length along whichthe light is transmitted through the solute in the solution in thechamber; and an output for outputting the light transmitted along theoptical path length, wherein the output light is received at a detectorand analyzed to determined the concentration of the solute in thesolution, and the optical path length of the chamber is selected tooptimize sensitivity of the detector, and wherein the optical pathlength of the chamber is related to the sensitivity of the detector bythe equations: min signal=c _(max)/10000(2−e ^((α(w2)*L))) c_(max)=968.8/L−α(w 2)*881.8 where min signal represents the minimumresolvable signal, c_(max) represents the maximum detectableconcentration, and L represents the optical path length of the chamber.40. The apparatus of claim 39, wherein the optical path length isselected to provide an adequate intensity of light to the detector. 41.The apparatus of claim 39, wherein the optical path length is selectedto provide an adequate resolution for the detector.
 42. The apparatus ofclaim 39, wherein the spectral distribution of the output light isanalyzed to determine the concentration of the solute within thesolution based on the spectral distribution.
 43. The apparatus of claim39, wherein the output light is detected by an array of detectors. 44.The apparatus of claim 39, wherein the concentration of the solute isdetermined based on a signature of the received light the concentrationof the solute.
 45. The apparatus of claim 44, wherein the signatureincludes at least one peak absorption wavelength, and the output lightis detected by a detector for each peak absorption wavelength.
 46. Theapparatus of claim 39, wherein the output light is detected by selectiveoptics and photosensitive elements.
 47. The apparatus of claim 46,wherein the wavelength selective optics include a diffraction grating.48. The apparatus of claim 46, wherein the wavelength selective opticsinclude filters.
 49. The apparatus of claim 46, wherein thephotosensitive elements include photosensitive diodes.
 50. The apparatusof claim 39, wherein the solute is chlorine, and the solution includeschlorine in water.
 51. The apparatus of claim 39, further comprising aninput for the solution filling the volume and an output for outputtingthe solution.
 52. The apparatus of claim 39, wherein the output light iscompared with a reference signal.
 53. The apparatus of claim 52, whereinthe reference signal is fed directly from the optical source to thedetector.
 54. The apparatus of claim 53, wherein the reference signal issubtracted from the detected signal to eliminate noise.
 55. Theapparatus of claim 39, wherein the concentration of solute in thesolution is determined in real time.
 56. The apparatus of claim 39,wherein the input light is provide by a controlled flashing lightsource.
 57. The apparatus of claim 51, wherein the flow of solution inand out of volume is controlled.
 58. The apparatus of claim 39, whereinthe detector sensitivity is in the parts per million (ppm) range.